You have three different colors/shapes/faces of objects on the screen, moving around, more trickling in. You do something (click? push off the screen? shoot?) to eliminate them. The goal is to get rid of them all without ever having one color/shape/face represent more than twice as much as any other.

It could be a cooperative game too.

You could decide whether to tell the player(s) how many of each color is left, or to make them judge.

Depends on what grade level you are. If you’re a freshman in high school like I am you’d probably want to go with something basic like invaders or brick breaker. But if you’re in college I think you should do a cool side-scroll runner like Canabalt except you have bosses that you race every 1000m that get faster with each one. You can also shoot the boss to slow him down, but he dodges the bullets by jumping or doing a special trick that has limits, like becoming a ghost so he can allow it to go through, but he can only do that once every 10 seconds and it has to power back up. Just a suggestion. That’s probably what I’d make if I knew how.
I also like platformers like Super Mario.

You wake up in a glass cube and a computer voice gives you this weird gun that shoots holes in the walls that take you to other dimensions. And tells you that you’ll be given cake if you solve all the upcoming puzzles…